IrOx nanostructure electrode neural interface optical device

ABSTRACT

An optical device with an iridium oxide (IrOx) electrode neural interface, and a corresponding fabrication method are provided. The method provides a substrate and forms a first conductive electrode overlying the substrate. A photovoltaic device having a first electrical interface is connected to the first electrode. A second electrical interface of the photovoltaic device is connected to a second conductive electrode formed overlying the photovoltaic device. An array of neural interface single-crystal IrOx nanostructures are formed overlying the second electrode, where x≦4. The IrOx nanostructures can be partially coated with an electrical insulator, such as SiO 2 , SiN, TiO 2 , or spin on glass (SOG), leaving the IrOx distal ends exposed. In one aspect, a buffer layer is formed overlying the second electrode surface, made from a material such as LiNbO3, LiTaO3, or SA, for the purpose of orienting the growth direction of the IrOx nanostructures.

RELATED APPLICATIONS

This application is a Divisional of an application entitled, OPTICALDEVICE WITH IrOx NANOSTRUCTURE ELECTRODE NEURAL INTERFACE, invented byZhang et al., Ser. No. 11/496,157, filed Jul. 31, 2006 now U.S Pat. No.7,494,840,

which is a continuation-in-part of an application entitled, IRIDIUMOXIDE NANOTUBES AND METHOD FOR FORMING SAME, invented by Zhang et al.,Ser. No. 10/971,280, filed Oct. 21, 2004, now U.S. Pat. No. 7,098,144.

The 11/496,157 application is also a continuation-in-part of anapplication entitled, IRIDIUM OXIDE NANOWIRE AND METHOD FOR FORMINGSAME, invented by Zhang et al., Ser. No. 10/971,330, filed Oct. 21,2004, now U.S. Pat. No. 7,255,745.

All of the above-mentioned applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to integrated circuit (IC) fabricationand, more particularly, to an optical device with an iridium oxidenanostructure neural interface.

2. Description of the Related Art

FIGS. 1A through 1C depict the placement of optical electrodes, usingepiretinal and subretinal approaches (prior art). Studies show thatblind patients with retinitis pigmentosa and masular degeneration canobserve a visual percept induced by the direct electrical stimulation ofthe retina. Recently, retinal prosthesis development has progressedalong two directions. The epiretinal approach places electrode in thevitreous fluid, attached to the surface of the retina, while thesubretinal approach places electrodes on the outside of retina, wedgedbetween the photoreceptors and the pigment epithelium. FIG. 1C shows theelectrode positions for both type of retina prosthesis.

One major difficulty with the epiretinal approach is that the tissue hasa very high resistance to electrical stimulation. This high electricalresistance is due to the fact that the retinal is covered by the innerlimiting membrane, similar to the blood-brain barrier, and it isimpermeable to many types of ions. Since electrical stimulation dependson electrical current building an electrical field across the targetnerve cell, a high-resistance barrier in the stimulation pathwayprevents current passage. Thus, current is diverted away to other,lower-resistance tissues. To compensate for this loss, the epiretinalapproach requires a much larger charge at the electrode surface, toachieve the same stimulation level on the target tissue as aconventionally functioning eye. In human trials, a net charge of ˜1 uC(micro-Coulomb) is required for the epiretinal approach, a very highcharge for neural stimulation, when compared to 50 nC, which is thelevel typically required for central nervous system stimulation andsubretinal stimulation.

One way to overcome the high-impedance barrier of the epiretinalapproach is to penetrate the inner limiting membrane. Arrays of sharpelectrodes have been fabricated from silicon and have been used for thispurpose. Recently, new techniques promise nano-scale piecing wireelectrodes, which can be formed by electrodeposition on a glasssubstrate contoured to fit the retina.

The subretinal prosthesis avoids the high-impedance barrier problem, byinstalling electrodes behind the retina. The electrodes are also veryclose to the bipolar cells, allowing easy (low-charge) stimulation. Lowthresholds in the range of 2.8 to 100 nC/cm² have been reported.However, 178 uC/cm² is a more realistic number. Subretinal prosthesesrequires that all components be fitted behind the retina, with thecircuits integrated with the electrodes. Power is transferred into theeye via light, which in theory is received by integrated photovoltaiccells, to activate the circuitry. The power output needs to be strictlycontrolled, since heat dissipation is limited in the subretinal space,and overheating can easily damage the retina.

One technical challenge is the trade-off between electrode density andstimulation charge. Although the total charge injection required toelicit a visual percept is fixed, the maximum charge an electrode candeliver is limited by its surface area. Surpassing this charge densitythreshold generates undesirable and irreversible electrochemicalreactions. In order to elicit a visual percept, the charge densityrequirement dictates that the electrode must have a total surface areaof 1 mm². When considering the limited area of the retinal implant, thisconstraint translates into a resolution of 5×5 (25 pixels). However, avisual resolution of at least 25×25 pixels (625 pixels) is desired forrecovery of functional sight.

To inject more charge without hydrolysis, an electrode made from amaterial with a higher injection limit can be used, such as iridium,oxide (IrOx). Compared to Pt, an IrOx electrode can inject much morecharge for a given voltage swing, by cycling iridium through manyoxidation states. Because iridium can exist in many valence states withan insignificant change in atomic size, an iridium electrode can cyclefrom metallic form (Ir) to higher oxidized form (IrO₄) reversibly,allowing it to have a high charge-injection limit of 1 mC/cm². Usingcyclic voltammetry with scan rates of 0.06V/s or slower, a chargeinjection of >25 mC/cm² can often be obtained. This behavior isattributed to IrOx porous structure, which requires ionic species todiffuse into deep recessed regions to access the full surface area, aswell as due to its many oxidation states, requiring the completion ofone state change before proceeding to the next reaction. IrOx isespecially ideal for applications with slower stimulation waveforms. Forneural stimulation, a current pulse longer than 200 us should beemployed. Furthermore, IrOx needs to be biased so that Its oxidationstate Is between Ir3+ and Ir4+ to prevent dissolution that occurs atmetallic or higher oxidation states.

A second approach to increase electrode charge-injection is to increasethe surface area. The surface area of an electrode is a strong functionof its geometry. The area of a solid post electrode can be increased bya factor of 10 easily if it is formed into an array ofnanowires/tubes/rods.

FIG. 2 is a schematic diagram modeling the interface between anelectrode and chemically active solution (prior art). Electrodes passcharge mainly through two mechanisms: faradic reactions and capacitivecharging. Capacitive charging is the accumulation of charge at theinterface between electrons in a metal electrode and ions adjacent tothe electrode in a solution, and is represented by C_(E). A faradicreaction is the transfer of electrons with ions in a solution by a redoxreaction of metal species, and is represented by R_(E). Both thecapacitive and faradic components increase linearly with the electrodearea because more charge can accumulate at the interface area and betransferred through chemical reactions by increasing the size of theelectrode. This larger electrode can be modeled as having a smallerR_(E) and bigger C_(E), leading to an increased electrode current at agiven potential. Either pulses of constant voltage or current can beused for electrical stimulation. Most frequently, pulses of constantcurrent are used.

FIG. 3 is a partial cross-sectional view comparing a conventional flatelectrode with an electrode array (prior art). Micro-machinedneural-stimulating electrode array technology has also been researched.The micro-machined electrode has the advantage of providing additionalsurface area to decrease the current density, while increasing theelectrode density and avoiding material corrosion. However, a key issueto be resolved is the fabrication of an electrode array that can conformto the concave shape of the foveal pit. For example, such as array wouldneed to be formed on a flexible substrate (e.g., polyimide).

Another limitation associated with micromachining technology is size, asthe individually machined electrodes cannot be made to a nano-sizeresolution. Even if a template of nano-sized structures could bemicro-machined, plating an array of nanostructures, with a noble metalfor example, in a sufficiently high aspect ratio is a big challenge.

Micro-machined electrodes are normally formed from a thick film that isdeposited using a physical vapor deposition (PVD) process or electrodeplating. In either case, the resultant film, and micro-machinedelectrode post are also polycrystalline.

Single-crystal IrO₂ nanowires/rods/tips have a much longer life thanpolycrystalline IrO₂, due to their higher chemical reaction resistance.Single-crystal IrOx nanostructures also have a higher conductance thanpolycrystalline IrO₂, so they can pass through current more efficiently.However, it is difficult to form single-crystal IrO₂ films usingconventional PVD or electrode plating methods. IrO₂ nanostructures canbe formed using a solution method, but these structures have a lowmechanical strength and poor crystal quality. Vapor phase transportmethods can also be used to form IrO2nanostructures, but this processrequires high substrate temperature, and it is not suitable for use withglass and polyimide substrates.

A technology that can grow free standing highly crystallizednanowires/tubes/rods array of IrO_(x) on selected areas of electrodewould be useful.

It would be advantageous if an optical neural interface could befabricated using an IrOx nanostructure array formed on a flexiblesubstrate.

It would be advantageous if low substrate temperature chemical vapordeposition. (CVD) methods could be used to directly form high-densitysingle-crystal IrO₂ nanowires/rods/tip electrode arrays.

SUMMARY OF THE INVENTION

The present invention describes a IrOx nanowires/nanotubes/nanorod arrayused as a stimulation electrode powered by a photovoltaic device (e.g.,a photodiode), which is response to visible light, infrared (IR) light,or both. The IrOx electrode array device has applications as a retinalprosthesis and, more generally, in any medical or biological applicationrequiring a neuron interface to either receive messages from, or delivermessages to the neurons.

Accordingly, a method is provided for forming an optical device with anIrOx electrode neural interface. The method provides a substrate andforms a first conductive electrode overlying the substrate. Aphotovoltaic device having a first electrical interface is connected tothe first electrode. A second electrical interface of the photovoltaicdevice is connected to a second conductive electrode formed overlyingthe photovoltaic device. An array of neural interface single-crystalIrOx nanostructures are formed overlying the second electrode, wherex≦4. In one aspect, the IrOx nanostructures have a proximal end attachedto the second electrode, and are partially coated with an electricalinsulator, such as SiO₂, SiN, TiO₂, or spin on glass (SOG), leaving theIrOx distal ends exposed.

Rather than micro-machining, the array of IrOx nanostructures can beformed using a chemical vapor deposition (CVD) process to grow IrOxnanostructures from a surface of the second electrode. For example, theCVD process introduces a(methylcyclopentadienyl)(1,5-cyclooctadiene)iridium(I) precursor anduses a metalorganic chemical vapor deposition (MOCVD) process to growIrOx nanostructures from the second electrode surface. In one aspect, abuffer layer is formed overlying the second electrode surface, made froma material such as LiNbO3, LiTaO3, or SA, for the purpose of orientingthe growth direction of the IrOx nanostructures. In another aspect, agrowth promotion film is formed over the second electrode, made from amaterial such as Ti, Co, Ni, Au, Ta, polycrystalline silicon (poly-Si),SiGe, Pt, Ir, TiN, or TaN.

Additional details of the above-described method and an optical devicewith an IrOx electrode neural interface are presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C depict the placement of optical electrodes, usingepiretinal and subretinal approaches (prior art).

FIG. 2 is a schematic diagram modeling the interface between anelectrode and chemically active solution (prior art).

FIG. 3 is a partial cross-sectional view comparing a conventional flatelectrode with an electrode array (prior art).

FIG. 4 is a partial cross-sectional view of an optical device with aniridium oxide (IrOx) electrode neural interface.

FIG. 5 is a partial cross-sectional view showing a variation of theoptical device of FIG. 4.

FIG. 6 is a partial cross-sectional view detailing a first aspect of anIrOx nanostructure.

FIG. 7 is a partial cross-sectional view detailing a second aspect of anIrOx nanostructure.

FIG. 8 is a partial cross-sectional view showing a variation of theoptical device of FIG. 4 or FIG. 5.

FIGS. 9A and 9B are drawings depicting variations in the orientation ofIrOx nanostructures with respect to the surface from which they aregrown.

FIG. 10 is a partial cross-sectional view of the optical devices ofFIGS. 4 and 5 configured in a device array.

FIG. 11 is a flowchart illustrating a method for forming an opticaldevice with an IrOx electrode neural interface.

DETAILED DESCRIPTION

FIG. 4 is a partial cross-sectional view of an optical device with aniridium oxide (IrOx) electrode neural interface. The device 400comprises a substrate 402 and a first conductive electrode 404 overlyingthe substrate 402. The substrate 402 can be a material such as glass,quartz, plastic, or a flexible polyimide. However, the device 400 is notlimited to just these exemplary materials.

A photovoltaic device 406 has a first electrical interface 408 connectedto the first electrode 404, and a second electrical interface 410. Asecond conductive electrode 412 overlies the photovoltaic device 406 andis connected to the second electrical interface 410. An array of neuralinterface single-crystal IrOx (x≦4) nanostructures 414 overlies thesecond electrode 412. As used herein, a nanostructure is a structurehaving nano-sized feature, as defined in more detail below, which mayalso be referred to as nanowires, nanotubes, nanorods, or the like. Dueto the fabrication methods, discussed below, the present inventionoptical device 400 can uniquely be made from single-crystal IrOxnanostructures having a high aspect ratio, on flexible substratematerials.

A number of photovoltaic devices are available for use in thisapplication. For example, the photovoltaic (PV) device 406 can be aphotodiode (not shown) with a pn junction formed in a Si substrate. Asshown in FIG. 4, the first electrode 404 is made from a transparentmaterial such as ITO, ZnO, a thin metal layer (e.g. Ag or An), SnO₂:F,or carbon, nanotubes. However, the device 400 is not necessarily limitedto just these exemplary materials.

Thus, the first electrode 404 passes light to the photovoltaic device406. The light (wavelength) may be filtered in passing through the firstelectrode 404, or in the photovoltaic device 406. Using the photodiodeexample, light can be filtered in response to the thickness of Sisubstrate separating the pn junction from the first electrode 404. Otherexamples of photovoltaic devices include conventional Si solar cells,amorphous-Si PIN or NIP structures, dye sensitized solar cells, or othercompound semiconductor solar cell structures, However, the presentinvention optical device 400 is not necessarily limited to any of theseexemplary photovoltaic devices 406.

The second electrode 412 is a material such as doped Si, Al, Mo, Ag,TiN, TaN, Pt, Ir, Pd, Ru, or Au. The second electrode 412 need not betransparent. Again, this is a list of exemplary materials, and otherconductive electrode materials are well known in the art.

As is well understood in the art, a complete PV device needs to have aconductive electrode connected to each side of a pn (or PIN) junction.The PV device 406 operates by building up voltage or passing a currentbetween the conductive electrodes (404/412). However, the electrode thatpasses through light must he a transparent conductive electrode (TCO).

The optical device 400 shown in FIG. 4 can be used in epiretinalprosthesis applications. An epiretinal approach would place the IrOxnanostructure array 414 in the vitreous fluid, attached to the surfaceof the retina. That is, the IrOx nanostructures are pointed toward theretina and light is passing through the opposite side of the device 400.

FIG. 5 is a partial cross-sectional view showing a variation of theoptical device of FIG. 4. In this aspect, the second electrode 412passes light to the photovoltaic device 406, and is made from atransparent material such as ITO, ZnO, a thin metal layer, SnO₂:F, orcarbon nanotubes. Then, the first electrode 404 can be made from dopedSi, Al, Mo, Ag, TiN, TaN, Pt, Ir, Pd, Ru, or Au.

The optical device 400 of FIG. 5 is configured for use in subretinalapplications. The electrodes are placed outside of retina, wedgedbetween the photoreceptors and the pigment epithelium. In this case thelight will come from the “top” electrode 412, the electrode interfacingwith the IrOx nanostructure array 414.

FIG. 6 is a partial cross-sectional view detailing a first aspect of anIrOx nanostructure. The average IrOx nanostructure 600 has an aspectratio in a range of about 1:1 to about 1:1000. As used herein, aspectratio is defined as the ratio of the nanostructure height 602, to thenanostructure diameter or width 604 at proximal end 606 (base) attachedto the second electrode 412. The IrOx nanostructures have an averageheight 602 in the range of about 10 nanometers (nm) to about 10micrometers (μm). The IrOx nanostructures have an average proximal enddiameter 604 in a range of about 10 nm to about 10 μm.

FIG. 7 is a partial cross-sectional view detailing a second aspect of anIrOx nanostructure. In this aspect, an electrical insulator 700partially coats the IrOx nanostructure 600, leaving the IrOx distal end702 exposed. The electrical insulator 700 can be a material such asSiO₂, SiN, TiO₂, or SOG. However, other insulating materials are alsoknown in the art.

FIG. 8 is a partial cross-sectional view showing a variation of theoptical device of FIG. 4 or FIG. 5. In this aspect, a controlled growthbuffer layer 800 is interposed between the IrOx nanostructures 414 andthe second, electrode 412. The buffer layer 800 is made from a materialsuch as LiNbO3 (LNO), LiTaO3 (LTO), or SA, and its use is explainedbelow.

Functional Description

One advantages of using a nanowire/tube/rod array in an optical deviceis that such as array can provide multiple electrical contacts at thecellular level, for electronically discriminating amongst individualcells or small groups of cells within a tissue or organ. Such an arraycan direct electrical signals to or from individual cells, or smallgroups of cells within such tissue or organ, especially neural tissuesand organs. Neurologists have long sought electrode devices that canestablish stable electrical contact with a large number of individualnerve fibers within a nerve. The ideal electrode device can be adaptedto the anatomy of the nerve so that it can penetrate the nerve in anondestructive fashion, in order to form focused electrical contactswith a very large number of individual nerve fibers. In order to makeelectrical contact with individual nerve fibers within a nerve, the useof a nanostructure array is a good design choice.

As a biocompatible electrode material, IrO_(x) has advantages over Auand Pt. Au quickly dissolves when a high potential is applied to it. Pthas the longest history of use as an electrode material and itscharacteristics are well understood. However, Pt cannot sustain as muchreversible faradic charge as IrOx, and it catalyzes water electrolysisat low voltages, limiting its charge-injection capability. Ir isassociated with many reversible oxidation states and, thus, has a highfaradic current. Further, because it relies heavily on faradicreactions, IrOx is slower in delivering the current.

The IrO₂ and IrOx nanostructures been successfully grown on Si,polysilicon, glass, and ITO-coated polyimide flexible substrates, toname a few materials. Selective deposition can be obtained usingrefractory metal nano-particles, such as Ti, Ni, Au, etc. The growthlength, density, and vertical orientation can be controlled bytemperature, pressure, flow, substrates and time.

FIGS. 9A and 9B are drawings depicting variations in the orientation ofIrOx nanostructures with respect to the surface from which they aregrown. In FIG. 9A, the IrOx nanostructure are grown from LNO (11-00),and in FIG. 9B, the growth surface is LNO (112-0). As can be seen fromthe figures, the orientation of the nanostructures is different. Meaningthat the material on which the nanostructures are grown, can be used asa growth control variable, IrO_(x) nanostructures grown on LNO (11-00)are about normal to the wafer surface (within 5°), while IrO_(x)nanostructures on LNO (112-0) are normal to the wafer surface withinabout 35° (within 30-40°). Other materials, such as SA and LTO have asimilar effect on orienting the nanostructures.

FIG. 10 is a partial cross-sectional view of the optical devices ofFIGS. 4 and 5 configured in a device array. IrO_(x) nanostructures canbe selectively deposited on a patterned conducting media, such as Si(doped Si), poly-Si (doped poly-Si), ITO, Ti, TiN, Ta, TaN, Al, Mo, Ag,etc., using CVD methods. The conducting media is connected with one ofthe electrodes (e.g., the second electrode of FIGS. 4, 5, and 8) of aphotovoltaic (photodiode) structure. The photovoltaic structure can bebuilt on a glass or flexible substrate such as polyimide. An array ofoptical devices is built balancing the need of image resolution againstthe isolation between individual optical devices needed to prevent thecrosstalk. The photovoltaic device can be any state of art technology,such as a conventional Si solar cell, an amorphous-Si PIN or NIPstructure, CIGS (CuIn_(1-x)Ga_(x)Se₂) structure, dye sensitized solarcell, or other compound semiconductor solar cell structures.Multi-junction cells can be used to increase the efficiency and obtainwider spectrum responses. Both vertical and lateral photovoltaicstructure can also be used. Between the cells, boles can be drilled tolet the nutrition pass through the electrode structure.

As noted above, the IrO_(x) nanostructures can also be coated with aninsulating media, using spin on methods for example, to enhance thestrength of the electrode array. This insulating layer has a thicknesssufficient to just cover, or expose the electrode tips for stimulatingthe bio tissue. Either a direct light energy source or an indirect lightenergy source (such as from LED) can be used to activate thephotovoltaic arrays.

FIG. 11 is a flowchart illustrating a method for forming an opticaldevice with an IrOx electrode neural interface. Although the method isdepicted as a sequence of numbered steps for clarity, the numbering doesnot necessarily dictate the order of the steps. It should be understoodthat some of these steps may be skipped, performed in parallel, orperformed without the requirement of maintaining a strict order ofsequence. The method starts at Step 1100.

Step 1102 provides a substrate. Step 1104 forms a first conductiveelectrode overlying the substrate. Step 1106 forms a photovoltaic devicehaving a first electrical interface connected to the first electrode,and a second electrical interface. Step 1108 forms a second conductiveelectrode overlying the photovoltaic device connected to the secondelectrical interface. Step 1110 forms an array of neural interfacesingle-crystal IrOx nanostructures overlying the second electrode, wherex≦4. As noted above, the IrOx nanostructures have an average aspectratio in a range of about 1:1 to about 1:1000.

In one aspect, Step 1110 forms IrOx nanostructures with a proximal endattached to the second electrode, and a distal end. Then, Step 1112forms an electrical insulator partially coating the IrOx nanostructures,leaving the IrOx distal ends exposed. For example, the electricalinsulator includes partially coating the IrOx nanostructures can beSiO₂, SiN, TiO₂, or SOG.

In another aspect, forming the array of IrOx nanostructures in Step 1110includes using a CVD process to grow IrOx nanostructures from a surfaceof the second electrode. For example, using a CVD process to grow IrOxnanostructures in Step 1110 may include substeps. Step 1110 a introducesa (methylcyclopentadienyl)(1,5-cyclooctadiene)iridium(I) precursor, andStep 1110 b uses a metalorganic chemical vapor deposition (MOCVD)process to grow IrOx nanostructures from the second electrode surface.More explicitly, introducing the(methylcyclopentadienyl)(1,5-cyclooctadiene)iridium(I) precursor in Step1110 a may include additional substeps (not shown). Step 1110 a 1initially heats the precursor to a first temperature in the range of 60to 90 degrees C. Step 1110 a 2 maintains the first temperature in thetransport line introducing the precursor. Step 1110 a 3 mixes theprecursor with a carrier gas, and Step 1110 a 4 establishes a finalpressure in the range of 1 to 50 Torr. Additional details of thefabrication process can be found in the parent applications listed abovein the “Related Applications” Section.

In another aspect, Step 1109 a forms a buffer layer overlying the secondelectrode surface, made from a material such as LiNbO3, LiTaO3, and SA.Then, forming the array of IrOx nanostructures in Step 1110 includesorienting the growth direction of the IrOx nanostructures with respectto the second electrode surface, in response to forming the bufferlayer.

In a different aspect, Step 1101 establishes a substrate temperature inthe range of 200 to 600 degrees C. Step 1109 b forms a growth promotionfilm with non-continuous surfaces overlying the second electrodesurface. Then, Step 1110 b uses the MOCVD process to grow IrOxnanostructures from the growth promotion film surfaces. Typically, thegrowth promotion film used in Step 1109 b has a thickness in the rangeof 0.5 to 5 nanometers (nm). The growth promotion film material can beTi, Co, Ni, Au, Ta, polycrystalline silicon (poly-Si), SiGe, Pt, Ir,TiN, and TaN.

An optical device with an IrOx electrode neural Interface, andcorresponding fabrication processes have been provided. Examples ofspecific materials, process steps, and structures have been, presentedto illustrate the invention. However, the invention is not limited tomerely these examples. Other variations and embodiments of the inventionwill occur to those skilled in the art.

1. An optical device with an iridium oxide (IrOx) electrode neuralinterface, the device comprising: a substrate; a first conductiveelectrode overlying the substrate; a photovoltaic device having a firstelectrical interface connected to the first, electrode, and a secondelectrical interface; a second conductive electrode overlying thephotovoltaic device connected to the second electrical interface; and,an array of neural interface single-crystal IrOx nanostructuresoverlying the second electrode, where x≦4.
 2. The device of claim 1wherein the array of IrOx nanostructures includes IrOx nanostructureshaving an average aspect ratio in a range of about 1:1 to about 1:1000.3. The device of claim 1 wherein the first electrode is made from atransparent material selected from a group consisting of ITO, ZnO, athin metal layer, SnO₂:F, and carbon nanotubes.
 4. The device of claim 3wherein the second electrode is a material selected from a groupconsisting of doped silicon (Si), Al, Mo, Ag, TiN, TaN, Pt, Ir, Pd, Ru,and Au.
 5. The device of claim 1 wherein the second electrode is madefrom a transparent material selected from a group consisting of ITO,ZnO, a thin metal layer, SnO₂:F, and carbon nanotubes.
 6. The device ofclaim 4 wherein the first electrode is a material selected from a groupconsisting of doped Si, Al, Mo, Ag, TiN, TaN, Pt, Ir, Pd, Ru, and Au. 7.The device of claim 1 wherein each IrOx nanostructures has a proximalend attached to the second electrode, and a distal end; and, the devicefurther comprising: an electrical insulator partially coating the IrOxnanostructures, leaving the IrOx distal ends exposed.
 8. The device ofclaim 7 wherein the electrical insulator partially coating the IrOxnanostructures is a material selected from a group consisting of SiO₂,SiN, TiO₂, and spin on glass (SOG).
 9. The device of claim 1 wherein theIrOx nanostructures has an average height in the range of about 10nanometers (nm) to about 10 micrometers (μm).
 10. The device of claim 1wherein the IrOx nanostructures have an average proximal end diameter ina range of about 10 nm to about 10 μm,
 11. The device of claim 1 furthercomprising: a controlled growth buffer layer interposed between the IrOxnanostructures and the second electrode, made from a material selectedfrom a group consisting of LiNbO₃, LiTaO₃, and SA.
 12. The device ofclaim 1 wherein the substrate is a material selected from a groupconsisting of glass, quartz, plastic, and flexible polyimide.